You should not file
Form IL-1040-X, Amended Individual Income Tax Return, until you receive a federal finalization notification from the Internal Revenue Service stating that they have accepted your change either by paying a refund or by final assessment, agreement, or judgment. If your federal change decreases your Illinois tax, and you want a refund, you must file Form IL-1040-X within two years plus 120 days of federal finalization. If your federal change increases your Illinois tax, you must file a Form IL-1040-X and pay any additional tax within 120 days of the federal finalization date, or you may be assessed a late-payment penalty. If your amended return is filed incorrectly and additional liability is due, we must issue you a notice of deficiency within two years from the date you filed Form IL-1040-X. If you fail to file an amended return when required, we may issue you a notice of deficiency at any time.
